I started using Time Machine to backup my iMac to a Seagate "Free Agent" external disk.
At some point (several months after starting), I started getting a screen message that said that Time Machine was unable to complete the backup. At the time that it stopped working, the Time Machine back up took approx 350GB. FWIW, when I Googled the Seagate Free Agent disk, they had lots of trouble with Macs.

I tried the Disk Utility, but could neither verify nor repair the external disk, (unmount errors). I had another external disk, so I went ahead an installed it. It was a Hewlett-Packard brand disk, so it was Windows formatted, but as soon as I installed it, the system re-formated it.

When Time Machine runs now, I get the Time Machine icon in the Notification Center with an Exclamation Point, and it says Back Up Delayed. When I open Time Machine there's about 75GB of stuff backed up.
I can run Disk Utility and it says the disk is OK.

I'm trying to figure out why I get the Time Machine/Exclamation Point-Back Up Delayed thing.
I'm also curious as to whether I'm really backing up what I think I am, since on the old disk, the Time Machine backup eventually took up 350GB, will it become that big eventually, or is the fact that its only 75GB indicative of some incomplete back up?